I wear a Camillus when at work, I frequently have to deal with line and knots. Primarily they are used for stubborn knots, by shoving the taper into a tight spot to force slack where it doesn't otherwise want to go. They also come in handy when splicing stranded line with eye-splices and the like. I have a small 'fid' which is a small spike that I use when doing 'fancy work' which is the nautical macrame.
There is also a knot called the Marlin Spike Hitch. It is basically a slip-knot that the spike is inserted into to make a handle to pull the line snug, afterward the spike is slipped out again. Aside from tightening onboard lashing, I think it has uses when slinging a hammock.
Most spikes that come on folders tend to be rather pointy, which can poke you sometimes. The spike on my Camillus is a little more forgiving and I have still been able to work it into knots on very small line.
When you start talking about a separate spike, Myerchin makes an elegantly shaped Stainless one that are liked by the folks I know who carry them.
C. S. Osborne also makes carbon steel spikes that range from 4" to 48" they are described as 'wire rope marlin spikes' but obviously the spike doesn't care what it's used on. I wear a 6" in a dual sheath (rig) with a modified skinner (trimmed to a sheeps-foot) when I sail on tall ships, and I love the 'ducks-bill' shape at the tip instead of a round point. (the 'ducks-bill' is essential when splicing wire rope)
Other spikes are available from J Adams of Sheffield, UK (with both a pencil and a chisel point, which looks a lot like a screw-driver), and Linder of Solingen, Germany (which is basically a rod with a small taper at the end) as well as from others.
This is a long winded explanation and probably more than you wanted to know about marlin spikes. It sounds like you are not at sea often, so my best recommendation would be to hie ebay and try to find an old Camillus, Ka-bar or Buck. The important thing with a folder is to get one that has a LOCKING SPIKE, many cheapies don't lock and you can hurt yourself if it folds up on you when you are not expecting it.
Good luck,
--Jack